Process for the electrolytic production of aluminum



PROCESS FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM Filed July 16, 1946L. FERRAND 2 SI-IEETS-SHEETI April 22, 1952 Wma lvlglllzll lg T b m 3 I3 8 E F f i T le]...

April 22, 1952 L, FERRAND 2,593,741

PROCESS FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM Filed July 16, 19462 SHEETSSHEET2 Patented Apr. 22, 1952 PROCESS .FOR THE ELECTROLYTICPRQDUCTION OF ALUMINUM Louis Ferraml, Paris, France Application July'16, 1946, Serial No. "683,902 In .France July 17, .1943

Section '1, Public Law 690, Augus't'8j1946 Patent expires July 17, 1963.4 Claims.

i The French patent, Ser. No. 962,781 filed by the applicant on the730th of .June 1942 for "A Method :for the Automatic Operation of Ovens:for Melt-Electrolysis andArrangements thereof A first-feature of theinventionconsistsiin corriinuously distributing the alumina into theanodic dissolution chambers and to convey it into the bath. with the aidof an auxiliary reducing gas fed at regulataole rate and pressure;

preferably, such distribution is effected through 4 calibratedapertures; the leak flow of conveyor "gas-moreover serves as means toreduce-the lumps of alumina.

.A characteristic modification of 'the invention consists indistributingthe alumina continuously we :an intimate mixture with the auxiliary re.ducing gas; said mixture may advantageously be distributed throughcalibrated tubes and oifices.

A-characteristic of theinvention that is a ,con-

sequence of the preceding ones consists in using the pressure of thereducing gas (methane or any other gaseous hydrocarbon) in thedissolution'chambers, or the pressure of the-mixture-of dissolvedmaterial with reducing gas, as a means to regulate the interpolardistance.

These various improvements will now be described in detail hereinafter,reference being had to the appended drawing in which:

Figures 1 and 2 are diagrammatic sectional 4 views, drawn to differentscal s, ofa first embodiment. v Y

FiguresB and 4 are secticnalviews of a modi fication.

-'anode m, on which the anode-carrying shaft and the electric leads areindicated in dotted lines. Secured in fluid-tight manner on top of saidchamber g by a nipple f '(Fig. 2) with :the interposition-of an asbestosgasket 7' is a reservoir T, which is also circular in sectionandintowhich the alumina to be dissolved in the bath is fed at regularintervals through a tube t1 branched oil from a 'manifold T1 throughwhich the alumina is conveyed pneumaticallyin Well known manner.

The upper end of nipple f carries a removable steel or gunmetal plate ndrilled with one or more calibrated orifices such as 0 designed to 'letalumina flow therethrough at the desired rate. Of course, the diameterof the hole or holes 0 should be ascertained experimentally with greatcare in order that with the mean head corresponding to the leveldifierential it between two successive loadings the rate of flow of thealumina shall be obtained in accordance with the load of theelectrolyticoven and the number of dissolution chambers available.

Said chambers-might have an oblong section or even'be replaced, inone'and thesame anode, by one single elongated and narrow chamber becometoo narrow A complementary arrangement is thus necessary.

Thesaidadditionalerrangement of the invention consists in the use of atube t2 connected with a manifold T2 amid a narrow central tube throug'hall of which aneutral or preferably a reducing gas may be flowed intothe chamberq, e. :g.':methane or "some other hydrocarbon gas.

Another embodiment of the invention whereby still :more importantadditional technical efiects ,Figure 5 illustrates applications tovScderberg :50 rcanbe secured is 'illustratcdrin Figs. 3 and 4.

anodes.

The arrangement illustrated in .Fig. 1 ,is .designedmore especially foraluminum production taken as an example. It comprises a dissolutionchamber q, circular in section, bored in the;

In the said embodiment, :the :alumina fwhich ,first tis de gassed in 5avacuum to strip it "from the air therein, is mixed intimately,'prior'tozits being introduced into the oven with the. aid of aninjector or ta :small .gas turbine orjsome other suitable apparatus thatis not comprehended in the invention, with a reducing gas flowed at suchrate that the atomic carbon resulting from its dissociation at thetemperature of the electrolysis will be suiiicient instead of carbonfrom the anode to sustain the secondary reactions of anodic oxidization.The very principle of this aspect of the invention, together with theselection of the reducing gases to be employed (as a rule readilydissociable hydrocarbons) are justified by established thermodynamicconsiderations and by the latest theories about aluminum electrolysis inthe melt, according to which the primary electrolysis does not takeplace at the expense of the alumina itself but does it at the cost ofthe components of the fiuorinated electrolyte to which said alumina isadmixed. The anodic oxygen itself thus would not be concerned in theconveyance of the current and would merely be a product of secondaryreactions.

For instance, with methane gas whose available energy is already low atC. (1-=l2,800 calories) and should be still lower if not null at 950 C.,it can be estimated that the reactions at the expense of the anodicoxygen, separately or simultaneously, are as follows:

ZC-l-Oz-e 2CO-1=105,260 calories (1) C-{-O2 C Oz--1=93,190 calories (2)2H2+o2+ 2H2o-T=ss',24o calories (a) or else,

81,815 1- 88,2l5 calories (6) for that portion of the methane gas thatescaped dissociation. One is aware that reactions (4) and (5) take placesecondarily, as the methane gas is being cracked, at 850 C.

Of course, such reactions, which all take place between gases or withhighly divided carbon, are likely to happen rather than such as wouldinvolve the much more compact carbon of the anode. An inoxidizable anodeis thus obtained without sacrificing the recovery of electromotive forcearising from the heat liberated by these exothermic reactions.

As to the excess gas (Hz or CH4) that would escape oxidization withinthe electrolyte, it will be burned as it comes into contact with thesurrounding air, which is to be entered upon. the credit-side of thethermo-chemical balance-sheet for the electrolytic reaction as a whole,this allowing to cut down power consumption materially provided thecurrent density employed is properly adjusted.

It should be mentioned that said methane gas, of which natural sourcesare to be found only in Rumania and in the United States, can beproduced most easily by availing ones self of the method for obtainingalumina from bauxites which consists in reducing the metal impuritiestherein with coal and then carburizing the alumina to obtain aluminumcarbide according to a known method. Said carbide stripped from themetal impurities thus reduced, by its combination with water or steam,will give methane gas and alumina according to the known reaction:

Methane gas is thus obtained as a paying by-.

product which in turn will furnish the reducing carbon necessary for theanodic reactions, and this, in chemically pure state although generatedfrom ordinary coal, without the necessity of resorting, as usual for theobtainment of pure aluminum, to high-purity and consequently costlycarbon varieties such as petroleum coke.

The methane gas is preparatorily re-heated if necessary by passing thesame through sole or Wall tubes such as described in the French additionNo. 46,398, dated Feb. 2 1935, to Patent No.

782,136 in order to facilitate its dissociation,

whereafter it is mixed with the alumina in the proportion of about 400liters per kilogram of the latter, after which the mixture isdistributed through the manifolds T3 and the tubes ta to the variousanodes through vertical orifices S, which are larger in number andsmaller in section than the dissolution chambers in order that a moresatisfactory dispersion of the gases below the anode surface may beobtained. Fluidtightness in the fit of said tubes is in thecorresponding orifices is secured similarly with the aid of ascrew-threaded nipple f and an abbestos gasket.

It will be appreciated that such method of constant distribution ofdissolved material is independent of the shape of the anode system andis applicable notably to continuous so-called Soderberg anodes. Figure 5is a sectional view of a Soderberg anode m provided with verticalaluminum tubes s' connected by screw-thread engagement at f with thetubes ii to be screwed in proportion as the anode is consumed and whichare surrounded with carbon c maintained by the aluminum sheath of theanode. Said tubes ii are connected with the manifolds T4 through whichthe mixture of alumina with reducing gas is led down to the bath. Itwill be observed that with the anode equipment described hereinbeforeand the cathode equipment described in the French patent filed by theapplicant on the 11th of August 1943 under the proc. No. 482,247, for

A Melt-Electrolysis Oven Designed for Very Small Voltage Drop andContinuous Running- Off it becomes possible to shut off the oven influid-tight manner with the aid of an arch V since the bath has nolonger to be taken care of.

The use of a neutral or a reducing gas to flush the alumina or to forman intimate mixture therewith according to one of the above-describedfeatures leads to a twofold result:

(a) With suitable adjustment of the gas pressure by means of a pressureregulator controlling the feed into each oven the level of the bath ineach dissolution chamber will sink until balance is obtained between thepressure of the gas within the chamber and the height H of liquid baththat surrounds the anode above the bottom of the latter. It will befound that such balance is reached or slightly exceeded as soon as themeter arranged on each anode indicates a very low rate of flow. Theoccurrence of such low rate of flow only at one anode indicates thatsame is higher than the adjacent "ones, and once said anode has been sethorizontal according to the procedure already described in the aforesaidpatent it will only be necessary to sink the same until leak flowappears at some other anode, and so on. The simultaneous existence ofsuch a leak flow at all the anodes in one and the same oven attests thatthey all are immersed by an equal amount in the bath, which means thatthe interpolar distance d between their bottoms and the metal layer d1is the same at all the anodes (Fig. 1 and even Fig. where two or moreSoderberg anodes are-provided).

A highly reliable way is thus afforded to adjust said interpolardistance instead of the rough methods usually resorted to. Incidentally,such condition is of capital importance in the efficient performance ofthe electrolysis.

That a preparatory adjustment of the interpolar equidistance isindispensable will be appreciated readily since, due to the fact thatthe pressure of the reducing gas or mixture in the supply line isconstant everywhere, it is necessary that the back-pressure below eachanode should be the same to ensure equality in the amounts of aluminafed to each of them.

(17) Where the alumina is fed in by gas flush, the leak flow (that canbe adjusted at any desired value) will also be efiective to drive away.

alumina lumps encased in the cryolite and to bring them below the activesurface of the anode into impoverished regions of the bath where theirdissolution will be accelerated by the turbulent action of the gasesfrom the electrolysis. In these conditions, with proper regulation of"the flow of alumina through the orifices 0 (Figs.

1 and 2), a practically constant percentage of alumina in the bath isobtained and the occurrence of the anode effect is avoided. Moreover,such conveyance effect cannot but be aided by the increase in weight ofthe superficial layers of the bath at the outlet of the dissolutionchambers as soon as the percentage of alumina exceeds with an attendantdrop in the melting point, and reaches 24%, which corresponds to thecryolite-alumina eutectic.

Last, the method of flushing or mixing the alumina with gas isadvantageous in that light alumina varieties now become available thatusually cannot be dealt with by hand-feed methods on account of theexcessive amountsof dust raised.

I claim:

1. The method of producing electrolytic aluminum in an oven having acarbon anode suspended in a layer of molten alumina electrolytecomprising dispersing finely powdered alumina in an atmosphere ofgaseous methane, and discharging the resulting suspension directly intothe space between the anode and the molten layer, the proportion ofalumina to methane in said suspension being such that the methane issuificient in amount to combine the entire oxygen content of eachparticle of alumina therein; whereby to obviate oxidation of the carbonanode.

2. The method in accordance with claim 1 in which the suspension isdischarged into said space through a duct in the anode.

3. The method in accordance with claim 2, in which the formation of saidsuspension takes place prior to introduction of the materials into saidanode duct.

4. The method in accordance with claim 2, in which the formation of saidsuspension is carried out within the anode.

LOUIS FERRAND.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 527,848 Gooch et a1. Oct. 23,1894 1,837,070 Roth Dec. 15, 1931 1,851,817 Dow Mar. 29, 1932 2,231,030Scofield et al Feb. 11, 1941 FOREIGN PATENTS Number Country Date 9,676Great Britain of 1901 318,431 Great Britain- Sept. 5, 1929 511,076 GreatBritain Aug. 14, 1939 115,749 Switzerland July 1, 1926

1. THE METHOD OF PRODUCING ELECTROLYTIC ALUMINUM IN AN OVEN HAVING ACARBON ANODE SUSPENDED IN A LAYER OF MOLTEN ALUMINA ELECTROLYTECOMPRISING DISPERSING FINELY POWERDED ALUMINA IN A ATMOSPHERE OF GASEOUSMETHANE, AND DISCHARGING THE RESULTING SUSPENSION DIRECTLY INTO THESPACE BETWEEN THE ANODE AND THE MOLTEN LAYER, THE PROPORTION OF ALUMINATO METHANE IN SAID SUSPENSION BEING SUCH THAT THE METHANE IS SUFFICIENTIN AMOUNT TO COMBINE THE ENTIRE OXYGEN CONTENT OF EACH PARTICLE OFALUMINA THEREIN; WHEREBY TO OBVIATE OXIDATION OF THE CARBON ANODE.